[Dovecot] pop3 behaviour

Hi!

I'v installed dovecot over um-pop3 today. Dovecot has only pop3 enabled. imap will be enabled later. But there is one issue - on some accounts, after receiving mail to Outlook Express, mail is not deleted from /var/mail/username mbox file, and on most it is deleted. Why it works in that way?

Best regards Krzysztof